This invention relates to nuclear magnetic resonance.
In recent times, a great deal of effort has been concentrated in methods of obtaining N.M.R. spectra from localized regions in space. Such methods are of particular interest in in vivo spectroscopy. A number of methods have been proposed, involving the application of field gradients, in combination with various sequences of Rf pulses. Such methods are in part successful, but suffer from the disadvantage that the resolution of the spectra obtained tends to be degraded by the use of the gradients.
A number of proposals have been made for localizing N.M.R. spectra, without the use of field gradients. For example, a method has been proposed by M. R. Bendall, (U.S. Pat. No. 4,486,709) and a further paper by M. R. Bendall (Journal of Magnetic Resonance, 59, 406-409, (1984)) involving the use of a sequence of pulses of different amplitudes, and phases. For example, three pulses may be used before each signal acquisition, having pulse angles of 2.theta., .theta., 2.theta. respectively and the experiment is repeated for various combinations of the phase of the first and third pulses, specifically, with phase shifts of 180.degree. applied to the first pulse, and of 90.degree., 180.degree., and 270.degree. are applied to the third pulse. The signals obtained from the eight individual signal acquisitions carried out with the various combinations of these phase shifts are then averaged with appropriate inversion of the receiver phase, and it is found that the signal obtained from various parts of the sensitive region of the receiving coil cancel, so that the actual signal sensed by the receiving coil is from a region localized in space.
The shorthand notation used to indicate the eight pulses in this sequence is as follows: EQU 2.theta.[.+-.x]; .theta.; 2.theta.[.+-.x,.+-.y]
where .theta. represents an arbitrary pulse angle of the Rf pulse used, .+-. indicates that the corresponding pulse is repeated with a 180.degree. phase shift, and .+-.x.+-.y indicates that the corresponding pulse is repeated with the three 90.degree. phase shifts (ie four pulses in all). When the phase of the third pulse is .+-.y, the resulting signal is subtracted from the average, i.e. the receiver phase is inverted. Pulse sequences of this kind are referred to herein as "depth" pulse sequences.
The basis of depth pulse schemes has been previously described and the prior art is summarised in the above-mentioned prior art.
We have now devised a number of further techniques and apparatus related to those disclosed in the publication referred to above, and have undertaken a theoretical analysis to enable further useful pulse sequences to be predicted.
As described in the above publications, depth pulse sequences were introduced to enable multipulse methods, such as inversion-recovery T.sub.1 and spin-echo techniques, to be applied with inhomogeneous rf coils, in particular surface coils, and to enable at least partial localization of the sample region from which NMR signals can be detected, i.e. the sample sensitive volume. Complete localization of the sample sensitive volume can be achieved using static or pulsed magnetic field gradients in combination with depth pulses, or by using separate transmitter and receiver coils, or multiple transmitter coils, in combination with depth pulse sequences (M. R. Bendall, J. M. McKendry, I. D. Cresshull, and R. J. Ordidge, Journal of Magnetic Resonance, 60, 473, 1984)). Phase cycled pulse schemes also enable the major types of heteronuclear multipulse NMR to be used in conjunction with inhomogeneous Rf coils separately from, or in combination with, sample localization (M. R. Bendall and D. T. Pegg, Journal of Magnetic Resonance 57, 337 (1984)). In all cases these methods can be combined with NMR imaging methods. The major area of application is in in vivo spectroscopy, and in summary, depth pulse schemes are quite comprehensive in their applicability and are poised to make an important contribution to the development of in vivo spectroscopy.
Two equivalent forms of depth sequences are disclosed, which may be described, using the above notation, as EQU 2.theta.; .theta.[.+-.x]; (2.theta.[.+-.x,.+-.y]).sub.n ; acquire signal [1]
and EQU 2.theta.[.+-.x]; .theta.; (2.theta.[.+-.x,.+-.y]).sub.n ; acquire signal [2]
The present invention is concerned only with the second form and in the following description of the present invention several simple conventions will be used.
Only one pulse in a depth pulse sequence permits conversion of initial z magnetization to detectable transverse magnetization. This is called the excitation pulse and is given a variable angle to signify the existence of Rf inhomogeneity. In sequence [1] above, the excitation pulse is the .theta.[.+-.x] pulse, but in sequence [2], and in all other depth pulse schemes described herein, the excitation pulse .theta. is recognizable as the only pulse whose phase is not cycled. The excitation pulse may be of arbitrary phase, but its phase will be represented hereinafter as x. Pulses having a magnitude of fractions, or multiples of .theta. may be used for specific purposes.
There are two major types of phase-cycled pulses which may be combined with the excitation pulse in a depth pulse scheme. The two types may be denoted, as first indicated above, as A[.+-.x] and A[.+-.x,.+-.y], where A indicates the pulse angle, which will normally be some multiple or fraction of the excitation pulse angle .theta. (for example 2.theta.) and [.+-.x] and [.+-.x,.+-.y] signify that the respective pulse is repeated in successive sequences with the defined phase variations. Receiver phase inversion is normally used whenever an odd number of .+-.y pulse phases is employed. The magnitudes of these pulses are commonly, as indicated above, 2.theta.. Fractions or multiples of 2.theta.[.+-.x] such as .theta./3[.+-.x] and 4.theta.[.+-.x] may alternatively be utilized. However such variations will be referred to as being "2.theta.[.+-.x)] type" pulses, and are covered by the following theory for 2.theta.[.+-.x].
In expressions such as (2.theta.[.+-.x,.+-.y]).sub.n, n signifies a succession of n 2.theta.[.+-.x,.+-.y] pulses.
In the prior art described above, depth pulse schemes contribute to sample localization by limiting the detection of NMR signals to sample regions where the pulse angle .theta. in the depth pulse sequence is between 90.degree..+-.45.degree., between 270.degree..+-.45.degree., between 450.degree..+-.45.degree., between 630.degree..+-.45.degree. and so on. Outside these regions, detected signal intensity is reduced to a few percent (say &lt;4%) of normal. The prior art also discloses that signal from regions where .theta. is between 270.degree..+-.45.degree. and between 450.degree..+-.45.degree. (i.e. "high flux" signals) can substantially be eliminated on-resonance.